This is the first application filed for the present invention.
Not Applicable.
The present invention relates to optical amplifiers and in particular to a method and system for controlling gain of an optical amplifier.
In optical telecommunications, erbium doped fiber amplifiers (EDFA) are commonly used for efficient signal amplification of optical signals on fiber optic spans without requiring optical-to-electrical and electrical-to-optical signal conversion. EDFAs are especially efficient when multiple signal channels of different frequencies are present on a fiber. Traditionally, EDFA amplifiers are statically configured, with fairly stable signal channels on the fiber. Increasingly, however, signal channels on fibers are switched in and out to meet dynamic market demands. This requires EDFA amplifiers to be reconfigured to maintain consistent signal levels. A control system is thus required for actively controlling optical amplifiers under rapidly changing network conditions.
Amplifier control systems are known in which an optical probe signal is used to measure amplifier gain, which in turn, is used to adjust the pump power to keep the gain of the amplifier constant. As is well known in the art, control constants in the feedback loop determine the stability, sensitivity and transient response of the control system. Control constant values are normally selected to prevent the system from reacting too slowly, or oscillating in response to changes. Normally, optical amplifier control systems use fixed control constants that have been chosen to ensure unconditional system stability. However, this does not provide fast enough operation under certain conditions. The frequency response of an optical amplifier typically varies under different operating conditions. As the number of signal channels increases or decreases, and the amplifier pump power varies, the frequency response of the amplifier changes. This in turn, affects control system stability. It is therefore highly desirable to have an adaptive system that is tuned to the actual operating conditions of the optical amplifier.
One strategy that has been suggested is to use a look-up table of control constant values. This would store optimized control constant values for each of a range of operating conditions. This technique has the disadvantage of requiring an accurate look-up table that would have to be very large, because there are a wide variety of network conditions that affect the frequency response of the amplifier.
Another possible solution is to use an adaptive control system which adjusts the control constant values in response to it""s own performance. This technique has the disadvantage of being complex to implement and sensitive to stability problems.
Accordingly, a cost effective system for actively tracking and controlling an EDFA amplifier remains highly desirable.
An object of the present invention is to provide a cost effective system for actively tracking and controlling an EDFA amplifier.
Accordingly, an aspect of the present invention provides a method of tracking dynamic characteristics of an optical amplifier. The method comprises estimating a gain of the amplifier using a first optical probe signal having a first predetermined modulation frequency above an expected corner frequency of the amplifier and estimating a corner frequency of the amplifier using a second optical probe signal having a second predetermined modulation frequency at or below the expected corner frequency of the amplifier.
Another aspect of the present invention provides a system for tracking dynamic characteristics of an optical amplifier, the apparatus comprising a gain estimating means for estimating a gain of the amplifier using a first optical probe signal having a first predetermined modulation frequency above an expected corner frequency of the amplifier and corner frequency estimating means for estimating a corner frequency of the amplifier using a second optical probe signal having a second predetermined modulation frequency at or below the expected corner frequency of the amplifier.
In embodiments of the invention, the gain at both the first frequency and second frequency may be measured by injecting an optical probe signal having a first power into an input of the optical amplifier, measuring an output power of the optical probe signal at an output of the optical amplifier and determining the gain from a difference between the output power and the input power of the optical probe signal.
In embodiments of the invention, the optical probe signal may be provided by a modulated broadband light source. This broadband light source may be a light emitting diode. The output power of the optical probe signal may be measured by a synchronous detector.